In man and other laboratory animals, urinary excretion of the histamine metabolite, imidazoleacetate, is increased and that of its conjugated metabolite, ribosylimidazoleacetate, decreased by salicylates. Since imidazoleacetate has been reported to produce analgesia and narcosis, its accumulation as a result of transferase inhibition could play a part in the therapeutic effects of salicylates. In order to determine the locus of salicylate action, we investigated the effect of anti-inflammatory drugs on imidazoleacetate phosphoribosyl transferase, the enzyme that catalyzes the ATP-dependent conjugation of imidazoleacetate with ribose-5-phosphate. As little as 0.2 mM aspirin produced 50 per cent inhibition of the rat liver transferase. In vivo, a 30 per cent decrease in the urinary excretion of ribosylimidazoleacetate has been observed with plasma salicylate concentrations of 0.4 mM. The enzyme was also inhibited by sodium salicylate but not by salicylamide, sodium gentisate, aminopyrine, phenacetin, phenylbutazone, or indomethacin. The last four drugs have been shown not to alter the excretion of ribosylimidazoleacetate when administered in vivo. Since both the drug specificity and inhibitory concentrations are similar in vivo and in vitro, it seems probable that the effect of salicylates on imidazoleacetate conjugation results from inhibition of imidazoleacetate phosphoribosyl transferase.